"Reality Storm: When Worlds Collide" crossed over Hero Games' Champions and Guardians Of Order's Silver Age Sentinels, co-produced by both companies. The adventure itself was dual-statted, but the book also included official conversion notes between the Tri-Stat d10 system and 5th edition Hero System.
The Spelljammer novels of The Cloakmaster Cycle demonstrated such a campaign, with most of the first book taking place on Krynn, only gradually introducing the concept of spelljamming to the hero. Later he visited two of less known locations on Toril. At least Forgotten Realms actively supported the meeting with other worlds by dropping mentions in its own sources.
D&D also did a crossover with Magic: The Gathering in the form of the Plane Shift series of free Campaign Settings which gave detailed descriptions of various planes from the Magic universe with D&D-rules. Later, the Guildmasters' Guide to Ravnica took this premise Up to Eleven by making a whole Campaign Setting Book for D&D that takes place on the plane of Ravnica from Magic.
West End Games' Paranoia had a trilogy of crossovers under the banner "Vulture Warriors of Dimension X". The first two adventures crossed over with Cyberpunk 2013 and Twilight2000, while the third adventure used time- and dimension-travel as a campaign hook for open-ended crossovers with other franchises.
Warhammer 40,000 and Warhammer were originally much the same (the former being the latter Recycled In Space), but drifted apart with further editions with only Chaos and the Warp linking them together. Nevertheless, they are occasionally hinted to be in the same universe, with futuristic items and artifacts finding their way into the Chaos Wastes, and Kaldor Draigo making an appearance as well.